Progress in the preparation and testing of glycine transporter type-1 (GlyT1) inhibitors

Clinically utilized antipsychotic agents share as a common mechanism the ability to antagonize dopamine D2 receptors and it is widely assumed that this activity contributes to their efficacy against the positive symptoms of schizophrenia. The efficacy of currently marketed antipsychotic agents on the negative and cognitive symptoms of this disease, however, is not optimal. One alternate hypothesis to the "dopamine hypothesis" of schizophrenia derives from the observation that antagonists of NMDA receptor activity better mimic the symptomatology of schizophrenia in its entirety than do dopamine agonists. Findings from this line of research have led to the NMDA receptor hypofunction (or glutamate dysfunction) hypothesis of schizophrenia, which complements existing research implicating dopamine dysfunction in the disease. According to the NMDA receptor hypofunction hypothesis, any treatment that enhances NMDA receptor activity may prove useful for the treatment of the complex symptoms that define schizophrenia. This idea is now supported by numerous clinical studies that have reported an efficacious response following treatment with activators of the NMDA receptor co-agonist glycineB site. One area of study, aimed at potentiating the NMDA receptor via activation of the glycineB site is small molecule blockade of the glycine reuptake transporter type 1 (GlyT1). Broadly, these efforts have focused on derivatives of the substrate inhibitor, sarcosine, and non-sarcosine based GlyT1 inhibitors. Accordingly, the following review discusses the development of both sarcosine and non-sarcosine based GlyT1 inhibitors and their current status as putative treatments for schizophrenia and other disorders associated with NMDA receptor hypoactivity.     

The GABA-glutamate connection in schizophrenia: which is the proximate cause?

Schizophrenia is a chronic, disabling psychiatric disorder that genetic studies have shown to be highly heritable. Although the dopamine hypothesis has dominated the thinking about the cause of schizophrenia for 40 years, post-mortem and genetic studies have provided little support for it. Rather, post-mortem studies point to hypofunction of subsets of GABAergic interneurons in the prefrontal cortex and the hippocampus. Furthermore, clinical pharmacologic, post-mortem and genetic studies have provided compelling evidence of hypofunction of a subpopulation of NMDA receptors in schizophrenia. In support of this inference, agents that directly or indirectly activate the glycine modulatory site on the NMDA receptor (the Glycine B receptor) reduce symptoms in chronic schizophrenia, especially negative symptoms and cognitive impairments. Electrophysiologic and pharmacologic studies suggest that the vulnerable NMDA receptors in schizophrenia may be concentrated on cortico-limbic GABAergic interneurons, thereby linking these two neuropathologic features of the disorder.     

Metabotropic glutamate receptors: potential drug targets for the treatment of schizophrenia

Schizophrenia is a debilitating chronic psychiatric illness affecting 1% of the population. The cardinal features of schizophrenia are positive symptoms (thought disorder, hallucinations, catatonic behavior), negative symptoms (social withdrawal, anhedonia, apathy) and cognitive impairment. Although progress in elucidating the aetiology of schizophrenia has been slow, new insights on the neurochemical and neurophysiological mechanisms underlying the pathophysiology of this illness are beginning to emerge. The glutamate/N-methyl-D-aspartate (NMDA) hypofunction hypothesis of schizophrenia is supported by observations that administration of NMDA glutamate receptor antagonists such as phencyclidine (PCP) or ketamine induces psychosis in humans; moreover, decreased levels of glutamate and changes in several markers of glutamatergic function occur in schizophrenic brain. Administration of PCP or ketamine to rodents elicits an increase in locomotion and stereotypy accompanied by an increase in glutamate efflux in several brain regions. Systemic administration of group II metabotropic glutamate (mGlu) receptor agonists suppresses PCP-induced behavioral effects and the increase in glutamate efflux. Activation of group II mGlu receptors (mGlu2 and mGlu3) decreases glutamate release from presynaptic nerve terminals, suggesting that group II mGlu receptor agonists may be beneficial in the treatment of schizophrenia. In addition, pharmacological manipulations that enhance NMDA function may be efficacious antipsychotics. Selective activation of mGlu5 receptors significantly potentiates NMDA-induced responses, supporting this novel approach for the treatment of schizophrenia. The glutamate hypothesis of schizophrenia predicts that agents that restore the balance in glutamatergic neurotransmission will ameliorate the symptomatology associated with this illness. Development of potent, efficacious, systemically active drugs will help to address the antipsychotic potential of these novel therapeutics. This review will discuss recent progress in elucidating the pharmacology and function of group II mGlu and mGlu5 receptors in the context of current hypotheses on the pathophysiology of schizophrenia and the need for new and better antipsychotics.     

Direct and indirect modulation of the N-methyl D-aspartate receptor

The dopamine hypothesis of schizophrenia has been a driving force in guiding both theories of pathophysiology of schizophrenia, and drug discovery efforts in this area. While this path has been fruitful in producing a deeper understanding of the disorder and a variety of antipsychotic drugs, it is generally recognized that targeting the D(2) dopamine receptor has multiple shortcomings. Recently, alterations in the glutamatergic system have been proposed to play a key role in the neurochemical disruptions underlying schizophrenia. In particular, the similarities between the symptoms of schizophrenia and the psychotomimetic effects of non-competitive N-methyl D-aspartate (NMDA) receptor antagonists such as phencyclidine have spurred interest in the possibility that an NMDA receptor hypofunctional state might underlie schizophrenia. In this review, we summarize the NMDA hypofunction hypothesis of schizophrenia, and focus on the NMDA receptor as a potential target for novel antipsychotic agents. Both modulatory sites on the NMDA receptor, as well as G-protein coupled receptors such as the muscarinic and metabotropic glutamate receptors that modulate the NMDA receptor, are potential targets for the development of novel compounds that could ameliorate the symptoms of schizophrenia.     

NMDA-induced phosphorylation and regulation of mGluR5

Glutamate regulates neuronal function by acting on ionotropic receptors such as the N-methyl-D-aspartate (NMDA) receptor and metabotropic receptors (mGluRs). We have previously shown that low concentrations of NMDA are able to significantly potentiate mGluR5 responses via activation of a protein phosphatase and reversal of phosphorylation-induced desensitization. While low concentrations of NMDA are able to potentiate mGluR5 responses, higher concentrations of NMDA are actually inhibitory. In this report, we show that NMDA receptors and mGluR5 are highly colocalized in cortical regions. We also show that in voltage-clamp recordings obtained from Xenopus oocytes expressing mGluR5 and NMDA receptors, high concentrations of NMDA (50-100 microM) that elicited large currents (>400 nA) caused an inhibition of mGluR5 currents. Additionally, agonist-induced phosphoinositide hydrolysis presumably mediated by activation of mGluR5, is inhibited by NMDA (30 microM and above). Additional data presented in this report suggest that the inhibitory effect of NMDA is caused by phosphorylation of mGluR5 at protein kinase C (PKC) sites since NMDA induces phosphorylation of the receptor as measured in a back phosphorylation assay.     

Phencyclidine and genetic animal models of schizophrenia developed in relation to the glutamate hypothesis

In humans, phencyclidine (PCP), a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, reproduces a schizophrenia-like psychosis including positive symptoms, negative symptoms and cognitive dysfunction. Thus, PCP-treated animals have been utilized as an animal model of schizophrenia. PCP-treated animals exhibit hyperlocomotion as an index of positive symptoms, and a social behavioral deficit in a social interaction test and enhanced immobility in a forced swimming test as indices of negative symptoms. They also show a sensorimotor gating deficit and cognitive dysfunctions in several learning and memory tests. Some of these behavioral changes endure after withdrawal from repeated PCP treatment. Furthermore, repeated PCP treatment induces some neurochemical and neuronanatomical changes. Recently, genetic approaches based on "the glutamate hypothesis of schizophrenia" have been used to develop animal models of schizophrenia. NMDA receptor subunit zeta1 knockdown, epsilon1 knockout (KO) and zeta1 point mutant mice exhibiting a hypofunction of NMDA receptors show hyperlocomotion, social behavioral deficit, sensorimotor gating deficit or cognitive dysfunction. Forebrain-specific calcineurin KO, neuregulin 1 heterozygous KO and lysophosphatidic acid 1 receptor KO mice can also serve as animal models of schizophrenia. These findings suggest that PCP and genetic animal models would be useful for evaluating novel therapeutic candidates and for confirming pathological mechanisms of schizophrenia.     

Discovery of positive allosteric modulators for the metabotropic glutamate receptor subtype 5 from a series of N-(1,3-diphenyl-1H- pyrazol-5-yl)benzamides that potentiate receptor function in vivo

This report describes the discovery of the first centrally active allosteric modulators of the metabotropic glutamate receptor subtype 5 (mGluR5). Appropriately substituted N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamides (e.g., 8) have been identified as a novel class of potent positive allosteric modulators of mGluR5 that potentiate the response to glutamate. An iterative analogue library synthesis approach provided potentiators with excellent potency and selectivity for mGluR5 (vs mGluRs 1-4, 7, 8). Compound 8q demonstrated in vivo proof of concept in an animal behavior model where known antipsychotics are active, supporting the development of new antipsychotics based on the NMDA hypofunction model for schizophrenia.     

The possible involvement of metabotropic glutamate receptors in schizophrenia

Glutamate disruption is thought to have a major role in schizophrenia brain processes, possibly involving NMDA hypofunction. The metabotropic glutamate receptors are distributed in brain regions related to schizophrenia and seem to affect glutamate release in a moderate way. Compounds modulating these receptors are being investigated in animal models of schizophrenia, in an attempt to discover new antipsychotics. This article reviews the current research data regarding the role of these receptors in schizophrenia animal models. It was found that more research was done on Group I and II metabotropic receptors while investigation of group III receptors is still trailing behind. Accumulating evidence shows that mGluR5 antagonists by themselves do not necessarily disrupt pre-pulse inhibition (PPI), but can exacerbate disruption of PPI caused by MK-801 and PCP, while positive modulation of this receptor has beneficial effects on these models of psychosis. Group II agonists are also showing beneficial effects in animal models. It seems that metabotropic glutamate receptor modulators could be developed into a novel treatment of schizophrenia by altering glutamate release, thus overcoming the putative NMDA hypofunction. Although the implications from these pre-clinical studies to human schizophrenia patients are premature, the data obtained with some compounds point to promising results for drug development. More studies, with agents active at other mGluRs in animal models and schizophrenia patients as well as with human subjects are needed in order to clarify the role of the metabotropic glutamate receptors in the pathophysiology and pharmacotherapy of schizophrenia.     

Ionotropic glutamate receptors as therapeutic targets in schizophrenia

Evidence implicating dysfunction of glutamatergic neurotransmission rests largely on the finding that antagonists of the NMDA subtype of glutamate receptor, especially the dissociative anesthetics like ketamine, can reproduce the full range of symptoms as well as the physiologic manifestation of schizophrenia such as hypofrontality, impaired prepulse inhibition and enhanced subcortical dopamine release. To test the hypothesis that schizophrenia may result from NMDA receptor hypofunction a number of clinical trials have examined the effects of agents that act on the glycine modulatory site on the NMDA receptor. Glycine, D-serine, and the partial agonist, D-cycloserine, have been shown to improve cognition and decrease negative symptoms in schizophrenic subjects receiving typical antipsychotics. Results with D-cycloserine suggest that clozapine may enhance glycine modulatory site occupancy. Preliminary results with an allosteric modulator of the AMPA subtype of glutamate receptor suggest enhanced cognitive functions in subjects treated with clozapine.     

Targeting metabotropic glutamate receptors for treatment of the cognitive symptoms of schizophrenia

Several lines of evidence implicate NMDA receptor dysfunction in the cognitive deficits of schizophrenia, suggesting that pharmacological manipulation of the NMDA receptor may be a feasible therapeutic strategy for treatment of these symptoms. Although direct manipulation of regulatory sites on the NMDA receptor is the most obvious approach for pharmacological intervention, targeting the G-protein coupled metabotropic glutamate (mGlu) receptors may be a more practical strategy for long-term regulation of abnormal glutamate neurotransmission. Heterogeneous distribution, both at structural and synaptic levels, of at least eight subtypes of mGlu receptors suggests that selective pharmacological manipulation of these receptors may modulate glutamatergic neurotransmission in a regionally and functionally distinct manner. Two promising targets for improving cognitive functions are mGlu5 or mGluR2/3 receptors, which can modulate the NMDA receptor-mediated signal transduction by pre- or postsynaptic mechanisms. Preclinical studies indicate that activation of these subtypes of mGlu receptors may be an effective strategy for reversing cognitive deficits resulting form reduced NMDA receptor mediated neurotransmission.     

Glutamate signaling in the pathophysiology and therapy of schizophrenia

Glutamatergic neurotransmission, particularly through the N-methyl-d-aspartate (NMDA) receptor, has drawn attention for its role in the pathophysiology of schizophrenia. This paper reviews the neurodevelopmental origin and genetic susceptibility of schizophrenia relevant to NMDA neurotransmission, and discusses the relationship between NMDA hypofunction and different domains of symptom in schizophrenia as well as putative treatment modality for the disorder. A series of clinical trials and a meta-analysis which compared currently available NMDA-enhancing agents suggests that glycine, d-serine, and sarcosine are more efficacious than d-cycloserine in improving the overall psychopathology of schizophrenia without side effect or safety concern. In addition, enhancing glutamatergic neurotransmission via activating the AMPA receptor, metabotropic glutamate receptor or inhibition of d-amino acid oxidase (DAO) is also reviewed. More studies are needed to determine the NMDA vulnerability in schizophrenia and to confirm the long-term efficacy, functional outcome, and safety of these NMDA-enhancing agents in schizophrenic patients, particularly those with refractory negative and cognitive symptoms, or serious adverse effects while taking the existing antipsychotic agents.     

Topiramate antagonizes MK-801 in an animal model of schizophrenia

The phencyclidine (PCP) model of schizophrenia suggests that N-methyl-D-aspartate (NMDA) receptor hypofunction and its consequences may play an important role in the pathophysiology of this psychiatric disorder. Moreover, the schizophreniform psychosis caused by PCP resembles schizophrenia in all of the relevant domains of psychopathology, especially negative symptoms and cognitive dysfunction. Because of interest in the PCP model and possible NMDA receptor hypofunction in schizophrenia, animal behaviors elicited by PCP and its analogues have been characterized. These preclinical models may serve to identify candidate compounds that possess therapeutic efficacy in schizophrenia. Ideally, negative symptoms and cognitive dysfunction would also serve as therapeutic targets for these novel medications. In the current study, the ability of topiramate to attenuate the severity of a specific behavior elicited by MK-801 (dizocilpine), a high affinity analogue of PCP was studied in mice. Topiramate was chosen because it addresses two of the predicted pathological consequences of NMDA receptor hypofunction. Specifically, topiramate potentiates GABAergic neurotransmission and antagonizes the excitotoxic actions of glutamate at the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate (KA) classes of glutamate-gated channels. Topiramate was shown to inhibit MK-801-elicited "popping" behavior in a complex dose-dependent manner.     

Significance of dysfunctional glutamatergic transmission for the development of psychotic symptoms

It has been postulated that disturbances in glutamatergic transmission may contribute to the pathophysiology of schizophrenia. This view is based on several findings: (1) the noncompetitive NMDA receptor antagonists, phencyclidine and ketamine, induce both positive and negative psychotic symptoms in humans, which closely resemble those observed in schizophrenia; (2) a number of animal studies have shown that neuroleptics that ameliorate symptoms of schizophrenia (e.g. clozapine) also inhibit the effects of NMDA antagonists; (3) postmortem and in vivo studies have revealed alterations in ionotropic glutamate receptors (NMDA, AMPA, KA) and their modulatory sites in schizophrenia; (4) compounds enhancing the function of NMDA receptors potentiate the antipsychotic effects of neuroleptics in schizophrenic patients.     

Glutamate and dopamine dysregulation in schizophrenia--a synthesis and selective review

The dopamine hypothesis of schizophrenia is the principal explanatory model of antipsychotic drug action. Recent discoveries extend our understanding of the neurochemistry of schizophrenia, with increasing evidence of dysfunction in glutamate and GABA as well as dopamine systems. In this review, we study the evidence for dopaminergic dysfunction in schizophrenia, drawing data from neurochemical imaging studies. We also review the NMDA receptor hypofunction hypothesis of schizophrenia as a supplementary explanatory model for the illness. We examine predictions made by the NMDA receptor hypofunction hypothesis and consider how they fit with current neurochemical findings in patients and animal models. We consider the case for glutamatergic excitotoxicity as a key process in the development and progression of schizophrenia, and suggest ways in which glutamate and dopamine dysregulation may interact in the condition.     

Contributions of the D-serine pathway to schizophrenia

The glutamate neurotransmitter system is one of the major candidate pathways for the pathophysiology of schizophrenia, and increased understanding of the pharmacology, molecular biology and biochemistry of this system may lead to novel treatments. Glutamatergic hypofunction, particularly at the NMDA receptor, has been hypothesized to underlie many of the symptoms of schizophrenia, including psychosis, negative symptoms and cognitive impairment. This review will focus on D-serine, a co-agonist at the NMDA receptor that in combination with glutamate, is required for full activation of this ion channel receptor. Evidence implicating D-serine, NMDA receptors and related molecules, such as D-amino acid oxidase (DAO), G72 and serine racemase (SRR), in the etiology or pathophysiology of schizophrenia is discussed, including knowledge gained from mouse models with altered D-serine pathway genes and from preliminary clinical trials with D-serine itself or compounds modulating the D-serine pathway. Abnormalities in D-serine availability may underlie glutamatergic dysfunction in schizophrenia, and the development of new treatments acting through the D-serine pathway may significantly improve outcomes for many schizophrenia patients.     

Positive modulators of AMPA receptors as a potential treatment for schizophrenia

There is a consensus that current treatments of schizophrenia do not offer satisfactory efficacy for negative and cognitive symptoms. Recently, the dopaminergic hyperfunction hypothesis of schizophrenia has been enriched by the addition of the glutamatergic hypofunction concept. Accordingly, agents enhancing glutamatergic transmission should provide benefit in psychosis. In fact, some preclinical studies suggest that positive modulators of alpha-amino-3-hydroxy-5-methyl-4 isoxazolepropionic acid (AMPA) receptors, previously developed mainly for dementia, may fulfill such expectations. These agents attenuate various biochemical or behavioral effects produced by amphetamine or methamphetamine and enhance the action of antipsychotics. More importantly, preliminary clinical studies with the most advanced member of this class, CX-516(Cortex Pharmaceuticals Inc), indicate beneficial action on negative and cognitive symptoms as an add-on treatment t o clozapine. If these observations are confirmed in a larger scale clinical trial, this approach could be a major improvement in the treatment of schizophrenia.     

The therapeutic potential of glycine transporter-1 inhibitors

While current antipsychotic medications are often efficacious for the positive symptoms of schizophrenia, there remains a critical need for compounds with improved tolerability and efficacy for the negative symptoms and cognitive dysfunction associated with this disease. There is a growing body of evidence suggesting that the potentiation of N-methyl-D-aspartate (NMDA) receptor function may be a useful approach for the treatment of schizophrenia. One proposed strategy for this potentiation is to increase synaptic levels of the neurotransmitter glycine by blocking the glycine transporter-1. Since glycine acts as a required co-agonist for the NMDA receptor complex; this approach allows an increase in the effectiveness of normal glutamatergic signalling at the NMDA receptor complex. Recent preclinical research, focused on the development and testing of novel glycine transporter-1 inhibitors, suggests that this approach may be feasible. Converging clinical evidence suggesting therapeutic efficacy following the potentiation of glycinergic activity further supports this approach. Clinical studies with novel glycine re-uptake inhibitors will provide critical information regarding the therapeutic utility and tolerability of this treatment for schizophrenia and other disorders associated with NMDA receptor hypofunction.     

The therapeutic potential of glycine transporter-1 inhibitors

While current antipsychotic medications are often efficacious for the positive symptoms of schizophrenia, there remains a critical need for compounds with improved tolerability and efficacy for the negative symptoms and cognitive dysfunction associated with this disease. There is a growing body of evidence suggesting that the potentiation of N -methyl-D-aspartate (NMDA) receptor function may be a useful approach for the treatment of schizophrenia. One proposed strategy for this potentiation is to increase synaptic levels of the neurotransmitter glycine by blocking the glycine transporter-1. Since glycine acts as a required co-agonist for the NMDA receptor complex; this approach allows an increase in the effectiveness of normal glutamatergic signalling at the NMDA receptor complex. Recent preclinical research, focused on the development and testing of novel glycine transporter-1 inhibitors, suggests that this approach may be feasible. Converging clinical evidence suggesting therapeutic efficacy following the potentiation of glycinergic activity further supports this approach. Clinical studies with novel glycine re-uptake inhibitors will provide critical information regarding the therapeutic utility and tolerability of this treatment for schizophrenia and other disorders associated with NMDA receptor hypofunction.     

Glycine transporter-1: a new potential therapeutic target for schizophrenia

The hypofunction hypothesis of glutamatergic neurotransmission via N-methyl-D-aspartate (NMDA) receptors in the pathophysiology of schizophrenia suggests that increasing NMDA receptor function via pharmacological manipulation could provide a new therapeutic strategy for schizophrenia. The glycine modulatory site on NMDA receptor complex is the one of the most attractive therapeutic targets for schizophrenia. One means of enhancing NMDA receptor neurotransmission is to increase the availability of the obligatory co-agonist glycine at modulatory site on the NMDA receptors through the inhibition of glycine transporter-1 (GlyT-1) on glial cells. Some clinical studies have demonstrated that the GlyT-1 inhibitor sarcosine (N-methylglycine) shows antipsychotic activity in patients with schizophrenia. Currently, a number of pharmaceutical companies have been developing novel and selective GlyT-1 inhibitors for the treatment of schizophrenia. A recent double blind phase II study demonstrated that the novel GlyT-1 inhibitor RG1678 has a robust and clinically meaningful effect in patients with schizophrenia. In this article, the author reviews the recent findings on the GlyT-1 as a potential therapeutic target of schizophrenia.     

Enhancement of social novelty discrimination by positive allosteric modulators at metabotropic glutamate 5 receptors: adolescent administration prevents adult-onset deficits induced by neonatal treatment with phencyclidine

Metabotropic glutamate-5 receptors (mGluR5), which physically and functionally interact with N-methyl-d-Aspartate (NMDA) receptors, likewise control cognitive processes and have been proposed as targets for novel classes of antipsychotic agent. Since social cognition is impaired in schizophrenia and disrupted by NMDA receptor antagonists like dizocilpine, we evaluated its potential modulation by mGluR5. Acute administration (0.63–40 mg/kg) of the mGluR5 positive allosteric modulators (PAMs), 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB) and ADX47273, reversed a delay-induced impairment in social novelty discrimination (SND) in adult rats. The action of CDPPB was blocked by the mGluR5 antagonist, 2-methyl-6-(phenylethynyl)-pyridine (2.5–10 mg/kg), and was also expressed upon microinjection into frontal cortex (0.63–10 μg/side), but not striatum. Supporting an interrelationship between mGluR5 and NMDA receptors, enhancement of SND by CDPPB was blocked by dizocilpine (0.08 mg/kg) while, reciprocally, dizocilpine-induced impairment in SND was attenuated by CDPPB (10 mg/kg). The SND deficit elicited by post-natal administration of phencyclidine (10 mg/kg, days?7–11) was reversed by CDPPB or ADX47273 in adults at week?8. This phencyclidine-induced impairment in cognition emerged in adult rats from week?7 on, and chronic, pre-symptomatic treatment of adolescent rats with CDPPB over weeks?5–6 (10 mg/kg per day) prevented the appearance of SND deficits in adults until at least week?13. In conclusion, as evaluated by a SND procedure, mGluR5 PAMs promote social cognition via actions expressed in interaction with NMDA receptors and exerted in frontal cortex. MGluR5 PAMs not only reverse but also (when given during adolescence) prevent the emergence of cognitive impairment associated with a developmental model of schizophrenia.